The origin of the 90 K nematic transition in the chalcogenide FeSe, which
displays no magnetic order down to T=0, remains a major puzzle for a unifying
theory for the iron-based superconductors. We analyze this problem in light of
recent experimental data which reveal very small Fermi pockets in this
material. We show that the smallness of the Fermi energy leads to a
near-degeneracy between magnetic fluctuations and fluctuations in the
charge-current density-wave channel. While the two fluctuation modes cooperate
to promote the same preemptive Ising-nematic order, they compete for primary
order. We argue that this explains why in FeSe the nematic order emerges when
the magnetic correlation length is smaller than in other Fe-based materials,
and why no magnetism is observed. We discuss how pressure lifts this
near-degeneracy, resulting in a non-monotonic dependence of the nematic
transition with pressure, in agreement with experiments.Comment: 5 pages, 3 figure
